This invention relates to the field of extrusion technology. In particular this invention pertains to extruders. More particularly, this invention relates to twin-screw extruders, which contain novel fractional elements. Salient features of the invention are that the known element geometry allows for constant tip angle for all elements. A new class of elements herein called fractional elements has been invented. The characteristic of these elements is that they can have different tip angles. The tip angle is a critical component in the design of extruders. These novel elements work as conjugate pairs while satisfying the needs of twin-screw extruders. The fractional elements are suitable for processes such as mixing, dispersing, alloying, blending, plasticising, kneading, devolatising and metering of the material.
Twin-screw extruders are known in the art. A number of varieties of such elements for twin-screw extruders have been patented. Scanning through patent literature pertaining to twin-screw extruders, it has been found that the following patent specifications are of relevance to this invention. The cited documents and detailed description of prior art In respect of twin-screw extruders are enumerated as under.
References have been made to the following patent specifications and the merit and demerit of these inventions are described in the following paragraphs.
U.S. Pat. No. 4,824,256
U.S. Pat. No. 4,131,371
European Patent Number 0037984
As known in the art, co-rotating twin-screw extruders have a long barrel (a.k.a. casing or housing) with two parallel bores that intersect each other. Two parallel shafts that are both driven in the same direction without any slip are placed in the bores. Processing segments are mounted one after another in a continuous chain on the shaft that transmits the rotary motion without slippage to the segments. The twin-screw extruder segments (a.k.a. compounding elements) consist of kneading elements (a.k.a. kneading disks or kneading blocks), conveying elements (a.k.a. screws or screw elements), mixing elements and other special elements. These carry out the various tasks of mixing, dispersing, kneading, blending, conveying, devolatising and metering of material that are fed in the extruder. Each element on one shaft while maintaining continuity with the adjacent elements on the same shaft has a corresponding conjugate element in the perpendicular direction on the other shaft. The characteristic of the elements is that of conjugacy (similar to two intermeshing counter-rotating gears). They virtually wipe each other as they rotate in the same direction.
The tip-angle is a critical component in the design of twin-screw extruders. The tip-angle for the known fully wiping co-rotating elements is given by the following equation (1)
t=xcfx80/nxe2x88x92(2 cosxe2x88x921r)xe2x80x83xe2x80x83(1)
where
xe2x80x98txe2x80x99xe2x80x94Tip angle
xe2x80x98noxe2x80x99xe2x80x94Number of lobes (or) flights (or) starts
xe2x80x98rxe2x80x99xe2x80x94Adjusted Center distance to Barrel Diameter ratio and
r=(cxe2x88x92xcex4c)/(bxe2x88x92xcex4b)xe2x80x83xe2x80x83(2)
where
xe2x80x98cxe2x80x99xe2x80x94Distance between the center of the two bores (Center Distance)
xe2x80x98bxe2x80x99xe2x80x94Diameter of the bores (Barrel Diameter)
xe2x80x98xcex4cxe2x80x99xe2x80x94Planned clearance (or) gap between the elements
xe2x80x98xcex4bxe2x80x99xe2x80x94Planned clearance (or) gap between the barrel and the element
Note xcex4c and xcex4b are typically small values.
It is evident that if the following condition (3) is satisfied, a lobe with a tip angle greater than zero can be formed.
cos (xcfx80/2n)xe2x89xa6r less than 1xe2x80x83xe2x80x83(3)
The ratio of the center distance to the barrel diameter therefore controls the maximum number of lobes that are possible in the extruder with acceptable tip angle. An extruder constructed with a ratio closer to 1 can have many lobes 30 with non-zero tip angle. Typical extruders have only two lobes (xe2x80x98rxe2x80x99=xcx9c0.8) or three lobes (xe2x80x98rxe2x80x99=xcx9c0.9). The end geometry has rotational symmetry with respect to the number of lobes. Higher number of lobes results in a smaller tip angle in the element. The surface of the various elements is obtained by cylindrical or helical or rotary transformation of the end geometry.
A smaller xe2x80x98rxe2x80x99 is preferred due to better efficiencies in the operation in terms of power consumption and output rates. However, the reduction in number of lobes leads to lower plasticising capacity in the kneading zone of the extruder for certain applications. U.S. Pat. No. 4,824,256 discloses new kneading elements, retaining the known geometry of twin-screw elements, designed with more number of lobes for an extruder that has a smaller xe2x80x98rxe2x80x99. This is achieved by using in the design a barrel diameter that is smaller than the actual barrel diameter until an acceptable
xe2x80x98rxe2x80x99 (greater that 0.866 for three lobes) is achieved. The element so designed with more lobes is then mounted in an eccentric manner with respect to the center of the shaft A group of such disks one after the other at a specific angle to one another forms a kneading element. These elements continue to be conjugate and are fully-wiping in nature.
Limitations of the Prior Art
The hitherto known twin-screw elements have equal tip angles for all lobes.
These limit the flexibility in designing the different type of elements that work in a twin-screw extruder. For example, the small tip angle in the eccentric tri-lobed kneading element leads to higher wear rate as only one of the tips operates closer to the barrel wall. Increasing the tip-angle would reduce the free volume available in the extruder. Further more, increasing tip-angle also makes the element become close to a circular shape. It is known from European Patent Specification No. 0037984 that an eccentric disk with a circular cross-section can be used. Such an element can be classified as zero lobed. However, the desired platicising action reduces with increased circularity and therefore increase in tip angle is not preferred.
Our invention tackles these limitations in the following manner. The known element geometry allows for constant Up angle for all elements. A new class of elements called fractional elements have been invented that can have different tip angles. These elements continue to work as conjugate pairs while satisfying the needs of twin-screw extrusion.
The objective of the invention is to provide co-rating twin-screw extruders with new element geometry that can provide a greater flexibility in design of individual elements. The end geometry of known elements is limited by constant Up angle for all lobes. The known end geometry are limited to just a few possibilities for a given center distance to barrel diameter ratio xe2x80x98rxe2x80x99.
The primary objective of the invention is to design and develop a novel twin-screw extruder, which contains novel fractional elements.
Another object of invention is to invent a novel twin-screw extruder, which has greater flexibility in design of individual components.
Another object of the invention is to invent a novel twin-screw extruder for achieving a perfect transition by ensuring that for each cross-section of the element on one shaft there exists a conjugate cross-section on the corresponding shaft that is fully wiping.
Further objects will be clear from the following description.
A twin-screw extruder having elements for processing thermoplastic, thermoset and other material, embodies a barrel with two parallel intersecting bores, two sets of co-rotating fully intermeshed extruder elements that are linked to each other in a non rotary manner forming a continuous chain, each operating in one of the bores, each screw being provided with multi-lobed elements that have geometry such that the tip angles are different for some of the lobes that range from two to infinity and these geometry are transformed in a cylindrical or helical or rotary manner with or without interruption to form various types of fractional elements for various operations defined in the specification.